Add missing FFI: onSpawn handler

[JordanMartinez]

I noticed the onSpawn handler is missing here.

Adding it is straight-forward though we should use EffectFn2 for this:

export function onSpawn(cp) {
  return function (cb) {
    return function () {
      cp.on("spawn", function () {
        cb();
      });
    };
  };
}